The present disclosure relates generally to a tubing hanger for use with a subsea wellhead, and in particular, a mechanism for positioning and locking a tubing hanger into a subsea wellhead.
Tubing hangers are employed in subsea wellheads used in, for example, oil and gas wells. The tubing hanger supports the tubing, or “string”, which extends down into the production zone of the well. The process of installing a tubing hanger into a wellhead generally involves positioning the tubing hanger on a landing seat in the wellhead using, for example, a running tool attached to the tubing hanger.
Movement of the tubing hanger inside the wellhead after installation is a known problem. Tubing hanger movement can be caused by, for example, torsional force applied to the tubing hanger due to thermal expansion and contraction of the tubing string. Excessive movement can change the orientation of the tubing hanger with respect to the wellhead, making it difficult to reinstall the running tool during subsequent operations or to subsequently install the subsea tree on the wellhead. Movement of the tubing hanger can also cause premature failure of the sealing system between the tubing hanger body and the wellhead housing, and the seals at the hydraulic and electric connectors between the tubing hanger and the subsea Christmas tree.
Various mechanisms for securing tubing hangers in wellheads have been devised in order to reduce movement of the tubing hanger in the wellhead. For example, locking mechanisms are often employed to lock the tubing hanger into place in the wellhead. In addition, means for preloading the tubing hanger in order to reduce undesirable axial and rotational movement of the tubing hanger have also been devised.
However, providing the desired preloading of the locked tubing hanger can be difficult to achieve when the landing seat of the tubing hanger inside the wellhead has uncertain axial position. The uncertainty of the axial position can be due, at least in part, to tolerance accumulations caused by the stacking of many components in the wellhead and debris that can accumulate on the landing seat during drilling operations.
To account for this uncertainty in axial seating position, tubing hanger designs have employed adaptive mechanisms in order to accommodate large dimensional variations and still achieve preloading. One typical form of this type of mechanism employs a locking ring with tapered inner surface, being pushed into the receptive profiles by applying a measured force to wedge a locking sleeve behind the locking ring. Because this type of locking ring relies on the friction between the tapered surfaces of the locking ring and the locking sleeve to maintain the locking ring in a preloaded locking state, it is necessary to employ additional locking (or anti-backoff) to prevent the loss of preload from the movements of the locking sleeve under vibration and other disturbance over long term field service. Moreover, the final axial position of the locking sleeve has large variation because the small taper angle used for maintaining the frictional self lock amplifies the manufacturing tolerance in diametric dimensions of the relevant components. Thus implementation of anti-backoff of the locking sleeve is often adaptive in nature and sometimes depending on friction itself. One example of a design that employs a lockdown mechanism with an actuating mandrel that includes an anti-backoff mechanism is disclosed in U.S. Pat. No. 6,516,875.
One design for a tubing hanger with a preloaded lockdown mechanism is disclosed in U.S. Pat. No. 5,145,006, issued to David R. June. In the June patent design, the tubing hanger is locked into place and then a torque ring is rotated to preload the locking mechanism. However, this design requires a tool, such as a mechanical torque tool, that can be run to the subsea wellhead to rotate the torque ring to its preloaded position. Further, the torque applied to provide the desired preload using the June patent design can be problematic. In deep water well completions, for example, applying torque at the top side over a long running string can be undesirable.
Other tubing hanger designs achieve preloaded locking with non-adaptive components, such as a locking ring with a non-tapered, cylindrical inner surface. Once the locking sleeve is forced into an axial position behind the locking ring, the preload is entirely determined by the deflections of the components within the load path that have controlled dimensional interferences and that are insensitive to the axial position of the locking sleeve. In one previous design, where a tubing hanger with such a locking mechanism was landed onto a shoulder with large axial position variation, a pre-installation measurement trip had to be made for each installation to determine the position of each landing shoulder so that the tubing hanger could be adjusted before installation to obtain the required dimensional interference. Such measurement trips not only add operational cost, which is significant in deep water application, but also introduce additional uncertainty.
Improved designs for locking a tubing hanger into a wellhead with preload would be a welcome addition in the art. The present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.